The present invention relates generally to electromechanical actuators, and specifically to a linear actuator having improved fault tolerance and positional control.
A number of approaches have been developed to manipulate the linear position of an object or device through the use of an actuator. Linear actuators are pervasive where the movement of very large loads is required. Linear actuation has traditionally been met by the use of hydraulic and pneumatic cylinders. Electromagnetic actuators are known, however, to provide increased performance in many aspects as compared to either hydraulic or pneumatic cylinders.
One drawback to the use of electromagnetic actuators is a certain degree of increased complexity, giving rise to increased concern over the reliability of such devices. Accordingly, certain electromagnetic linear actuators have incorporated fail-safe mechanisms of one type or another. As an example, U.S. Pat. No. 4,289,996 discloses a powered linear actuator having dual closed loop servo motor systems driving a screw jack. The dual motors drive the screw jack through differential gearing and each has an armature lock which functions automatically if a motor circuit fails thereby enabling the other motor to continue driving the actuator alone. Potentiometer feedback is applied to dual error amplifiers or polarized relays that compare the feedback position signal with the input command signal and drive separate motor energization channels.
U.S. Pat. No. 5,865,272 discloses a linear actuator having an output shaft having a pair of driven wheels mounted thereon. One of the driven wheels is rotatably mounted in a fixed plane and has a drive nut for an associated thread on the output shaft. The other drive wheel is rotatably fixed to the output shaft. An input shaft is in a side-by-side relationship with the output shaft and adapted to be rotated by a suitable power source. The input shaft provides a drive wheel for each of the driven wheels, with the ratio between each drive and driven wheel set being chosen to rotate the driven wheels at different speeds in the same rotational direction and thereby produce a controlled axial movement of the output shaft in a direction depending upon the relative rotation of the driven wheels. A fail-safe arrangement is provided in the form of a clutch between the drive wheels of the input shaft, a back-drive for the output shaft, and biasing means for affecting a back-drive.
U.S. Pat. No. 5,957,798 discloses an electromechanical actuator having a linear output for moving an external load, the actuator having at least two drive motors, a synchronizer connected to the outputs of the drive motors, a differential mechanism combining the outputs of the drive motors, and a quick release mechanism connected to the differential mechanism and the actuator output. The quick release mechanism releases support of the external actuator load in response to an internal actuator jam and maintains support of the external actuator load in response to an external actuator overload.
U.S. Pat. No. 6,158,295 discloses a linear actuator including a housing, a spindle rotatable in both directions, a threaded nut driving a piston rod, and a motor capable of driving the spindle through a transmission. A disengagement unit is arranged in the transmission for interrupting the connection between the motor and the spindle in case of operational failure, such as overloading of the spindle. The disengagement unit comprises a braking device adjustable with respect to the actuator housing to cooperate with a coupling device for control of the rotational speed of the spindle when it is disengaged from the motor.
Although each of these designs provides certain advantages, none of these designs provides a fully fault-tolerant linear actuation solution totally suitable for use in applications where life or safety is at risk. Each of these designs has its drawbacks, as will be appreciated by those of skill in the art. For example, as noted above, in any application in which a mechanical device, such as an actuator, is employed to perform a function, there is the potential and the risk of failure of the mechanical device and attendant loss of functionality. In certain situations, such failure may have only minor consequences. Wherever actuators are employed in applications in which life or safety are at risk, however, the consequences are much more severe. In high-stakes applications, such as the control of an aircraft control surface, disengagement of the actuator from the applied load is simply not an acceptable approach. Similarly, locking up the actuator with a brake would generally not be an acceptable approach in such an application. Accordingly, there is an unmet need to prevent sudden or catastrophic failure in the linear actuators employed.
Although electromechanical solutions offer definite advantages over the lower-technology hydraulic and pneumatic solutions often used in traditional linear actuation applications, the rugged simplicity of the fluid cylinder has made it tough to beat from a cost and reliability standpoint. Further, it is known that single point failures frequently occur in electromagnetic linear actuators. Where a linear actuator is susceptible to loss of function from a single point failure, the actuator could completely fail to operate in the event of such a failure. As noted above, this is an unacceptable situation in many applications.
The present invention solves the problems associated with current linear actuators. For example, in various embodiments, the systems of the present invention overcome the risk of failure by incorporating features enabling them to continue to operate under a partial or total fault on one side of a dual system. Thus, the present invention provides, in certain embodiments, fault tolerant duality in a compact, concentric, fully integrated module. This compactness and integration does not exist in any existing designs.
In accordance with one aspect of the present invention, a fault tolerant linear actuator is provided that incorporate velocity summing, force summing, or a combination of the two. In one embodiment, the invention offers a velocity summing arrangement with a differential gear between two prime movers driving a cage, which then drives a linear spindle screw transmission. This embodiment is reconfigurable, but since it has only one transmission, it does not eliminate all possible single point failures. A second embodiment features two prime movers driving separate linear spindle screw transmissions (one internal and one external) in a totally concentric and compact integrated module. This system has no single point failures, which is desirable where failure would result in loss of life or high cost. A third embodiment uses two rotary actuators driving acme screws in place of the linear spindle screw transmission to make a very rugged high force system. A fourth embodiment is a force summing linear actuator based on a dual set of linear spindle screw drives summing forces through two clutches at the output attachment plate. A fifth embodiment uses an intermediate gear train between the input prime movers and the output spindle screws in order to better balance the torque/speed ratios and to enable a significantly higher motor speed than in the second embodiment. This two-stage reduction also allows for a significant reduction in the weight of the actuator.
The development of certain technologies makes it possible for the, electromechanical actuators of the present invention to surpass the performance of prior known designs in essentially every aspect of performance. As an example, the commercial availability of the roller spindle screw transmission is a significant step forward in performance. As another example, the development of modern highly-integrated circuits allows for increases in performance and reductions in cost at the same time. Using these and other technologies, the present invention not only offers high load capacity, it also offers very long life, high precision, and high velocity in a compact configuration and the potential for a high level of actuator intelligence.
Intelligence within the actuator itself makes it possible to balance operational priorities (speed, load, precision, smoothness, etc.) in real time. Intelligence within the actuator permits the system of the present invention to be highly fault tolerant. This fault tolerance depends on a full awareness of all the performance capabilities of the actuator in real time. This awareness requires access to a wide spectrum of sensors, each generating data quantifying performance criteria used to judge the actuator""s operation. Depending on the application, these performance criteria may be prioritized to meet in-situ operational goals. Here, the principal goal is to maintain operation under a fault. Depending on the operational requirements, the output of a faulty prime mover in an actuator may be quantified and used as a basis to temporarily raise the performance of the one or more fully-operational prime movers in order to make up for the loss of performance from the faulty prime mover. Alternately, the faulty prime mover may be taken completely out of service by braking it and xe2x80x9climping homexe2x80x9d using the remaining prime movers.
The teachings of the present invention may be employed in any application in which there is the potential for loss of life, a need to preserve a long mission in harsh environments without possibility of repair, or a potential for high cost resulting from sudden failure. This layered control should combine to give more precise operation under significant load disturbances.
Those skilled in the art will further appreciate the above-mentioned advantages and superior features of the invention, together with other important aspects thereof upon reading the detailed description that follows in conjunction with the drawings.